Probably:

a receiver's mail transfer agent it "bouncing" the mail back to the list
making it appear like it came from the original poster.

/P

At 12:34 10/07/2002 Monday, Erwin Weiblen wrote:
 >Hello all,
 >
 >I would like to assist the owners of a mailing list I am subscribed
 >to in avoiding the following wrong error message:
 >
 >Apparently since the list has moved to another server, a lot but
 >not all of the list members get the following error message each
 >time they send a mail to the list:
 >
 >--------------------------
 >
 >Subject: Rejected posting to [log in to unmask]
 >
 >Your message  is being returned to  you unprocessed because it
 >appears to have already been distributed to the HYPNOS  list. That is,
 >a message with identical text (but  possibly with different  mail
 >headers) has  been posted to  the list recently, either by you or by
 >someone else. If you have a good reason to resend this message  to the
 > list (for instance  because you have  been notified  of a hardware
 >failure with  loss of data), please  alter the text of  the message in
 >some way and resend  it to the list. Note that altering  the
 >"Subject:" line or adding blank lines at  the top or bottom of the
 >message  is not sufficient; you should  instead add  a  sentence or
 >two  at  the top  explaining  why you  are resending the  message, so
 >that the  other subscribers understand why  they are getting two
 >copies of the same message.
 >
 >--------------
 >
 >Despite these error messages the mails are distributed correctly. So
 >it is not a major problem but still irritating.
 >
 >Even more, from time to time there are mails in the list which appear
 >twice identically.
 >
 >Any idea what to do? Thank you very much in advance!
 >Erwin
End Reply